The most conventional calendar timepieces are described in the technical literature of horology.
The function of a so-called perpetual calendar device is to determine the number of days in the current month, and, more specifically, in the month of February. The perpetual mechanism is an approximate notion: most commercially available mechanisms are simple leap year mechanisms, either using a 48-notch month cam, or a month cam with 12 positions, wherein the position of the month of February comprises a leap year mechanism comprising a Maltese Cross or similar element, to name the most common devices.
The very specific management of end-of-century years, and quadricentennial years, imposed by the Gregorian calendar, makes these applications very rare, only end-of-century years are actually found in a few watches considered to be very highly complicated watches. Quadricentennial years and quadrimillennial years are only encountered in astronomical clocks, including the Strasbourg Cathedral clock perfected by Schwilgué in the XIXth century.
The design of a perpetual calendar device meets with two difficulties:                how to take account of the specificities of the type of calendar concerned and translate this into the form of a timepiece mechanism, and        how to update such a mechanism in case of stoppage. Updating is often so complex that the timepiece must never be stopped, as is the case of astronomical clocks for buildings. Even in the case of the most basic version of a perpetual Gregorian calendar that simply manages leap years in four-year cycles, any updating is accomplished by a large number of operations, up to 47 manoeuvres to arrive at the right year and the right month, which results in wear of the mechanisms.        